I. Field of the Invention
The present invention relates generally to the use of selective excitation in conjunction with driven free precession (DFP) to form nuclear resonance (NMR) images. In particular, the present invention relates to the combination of the aforenoted two NMR techniques to effect the selection of a plane section, which is subsequently reconstructed from multiple angle projections by known multiple angle reconstruction techniques such as are used in X-ray computed tomography. Alternatively, the plane section can be reconstructed using known Spin Warp, or Fourier Transform Zeugmatrography, methods to produce a map of the spin density, or a combination of spin density and relaxation times of the materials in the selected plane.
II. Description of the Prior Art
NMR imaging as a medical diagnostic tool offers a number of important advantages over the various other means available for probing the human body. The most significant of these advantages result from the completely non-invasive nature of the technology, and the ability to obtain spatially encoded specimen data with a high degree of precision. Additionally, NMR has minimal, if any, hazards for either patients or operators of the apparatus; and perhaps most importantly, NMR image intensities are increasingly being found to be sensitive to various disease states. Clinical studies now underway are noting that the relaxation time of malignant tissues are in general longer than those of the tissues of origin. This property is apparently not unique to cancerous tissue, but rather is indicative of the changes in molecular level structure of water associated with certain disease states. Other pathologies of NMR imaging include hydrocephalis; a carotid artery aneurysm; edema associated with kidney transplants; and liver cirrhosis.
The interested reader is referred to a recent article titled "Spin Warp NMR imaging and applications to human wholebody imaging" by William A. Edelstein et al, Physics in Medicine and Biology, 25, pp 751-756 (1980); and further to "Nuclear magnetic resonance: beyond physical imaging" by Paul A. Bottomley, IEEE Spectrum, Vol. 20, no. 2, pp 32-38, (1983). More complete treatments of basic NMR concepts are provided in a recent text edited by Leon Kaufman et al., "Nuclear Magnetic Resonance Imaging and Medicine", Igaku-Shoin, New York and Tokyo (1981); and also in an earlier text by Thomas C. Farrar et al., "Pulse and Fourier Transform NMR, An Introduction to Theory and Methods", Academic Press, New York (1971).
In general, known NMR techniques for imaging of body tissue have tended to be of somewhat limited image quality, spatial resolution, and have required comparatively long patient exposure times to complete. In view of these technical and medical considerations, it is clearly of importance to improve NMR imaging technology by all available means. In particular, signal to noise ratios need to be increased, imaging times need to be shortened, spatial resolution needs to be enhanced, and imaging of the transverse and/or longitudinal relaxation times need to be accomplished. Many of these factors are not mutually exclusive. As a result, there has been a tendency to trade an improvement in one factor to the detriment of others. These trade offs are not always salutory, further pointing up the need for basic improvements in NMR methods and apparatus.
The coined term Zeugmatography is presently being used to cover an increasing range of NMR techniques wherein static magnetic fields (to produce polarization of nuclei) are combined with field gradients (to spatially encode the sample volume of interest) and with RF fields (to spatially reorient polarized nuclei) to achieve a wide range of objectives, including imaging. In the recept past, the technical and patent literature have burgeoned reporting results of successive advances in the fields. While the field has progressed steadily, certain intrinsic drawbacks have heretofore precluded the use of NMR high resolution imaging in medicine. Chief among these are comparatively slow relaxation times of human tissue, and body motion due both to inherent movements within the body as well as the difficulty of keeping the body stationary for long periods of time.
Biological tissue is known to have longitudinal (or spin-lattice) relaxation times T.sub.1, and transverse (or spin-spin) relaxation times T.sub.2, in the range of 0.04 to 3 seconds. Both of these times constants are exceedingly long as compared to the speed of the instrumentation presently available to process NMR signals. Also, high resolution imaging requires a large number of pixels, each of which may be the result of a complete NMR pulse projection, where each NMR projection is at least influenced by if not limited by these long time constants.
One of the fundamental limitations to NMR imaging of the whole human body is the relatively low signal to noise ratio of the NMR signals coming from body tissues. DFP is a good way to get maximum information gathering rate from an NMR experiment, and thus is attractive for imaging purposes.
Heretofore DFP has been used to make tomographic-like section images by applying a sinusoidally oscillating gradient perpendicular to the selected slice in the sample of interest. (See, for example, H. R. Brooker and W. S. Hinshaw, J. Mag. Res. 30, 129-131.) As described, the DFP signal is averaged over time, and the signal produced by spins distant from the null plane (of the oscillating gradient) average to zero. This technique has a number of disadvantages, a primary one being that in order to get a desired plane thickness and signal to noise ratio, there is a minimum time over which the average must be taken in order to minimize the signal from outside the selected plane. Also, the thickness of the plane at any location depends on the ratio of the longitudinal/transverse relaxation times (T.sub.2 /T.sub.1) of the sample; and under certain conditions there is a significant contribution from spins outside the slice of interest. The effect of these unwanted signals can be large and is only limited by the finite size of the sample and the finite spatial extent of the radio frequency fields employed. Additionally, there is no good way in these prior art methods to define an extended region perpendicular to the selected slice, or to obtain spatial information in the thickness direction of a slice.
Selective excitation has been used to select a plane for imaging. However, prior art schemes using selective excitation have required time delays approximately equal to (or greater than) the longitudinal relaxation time T.sub.1, which generally degrades the signal to noise ratio of the overall NMR data acquisition process. No prior art schemes are known which combine DFP and selective excitation and produce imaging pulse sequences.
The present invention teaches the use of new NMR imaging sequences based on advantageously combining known sequences in particular modes, and is specifically directed to produce a substantial improvement in the heretofore available image signal to noise ratio by maximizing the information gathering rate from an NMR pulse sequence; and/or produce a 2D or 3D image, responsive both to the spin density and to the longitudinal relaxation time T.sub.1 of the sample under study.